Recently, to increase protection and beauty of a display in flat panel display devices such as mobile phones and personal digital assistances (PDA), a thin plate-shaped cover glass is provided on a front surface of a display so as to form a region wider than an image display portion. Weight reduction and thickness reduction are required to such flat panel display devices, and to achieve those requirements, a cover glass used for protecting a display is also required to reduce its thickness. However, where the thickness of the cover glass is reduced, strength is decreased, and the cover glass itself may crack by dropping during the use or during carrying. Thus, there has been a problem that the primary function of protecting a display device cannot be performed.
For this reason, a conventional cover glass has increased its strength by chemically strengthening a glass plate to form a compressive stress layer on the surface thereof (for example, JP-A-2011-105598).
Since a flat panel display device is portable, it is considered that, among cases where a cover glass has fractured, there are a lot of cases where a stone or the like collides with a glass surface by dropping the device, and the cover glass fractures starting from a crack generated by the indentation. That is, high resistance to indentation, rather than bending strength, is required as strength of a cover glass.
Conventionally, to evaluate strength of such a cover glass, superiority or inferiority of a cover glass has been evaluated by pushing an indenter having relatively large tip angle, such as Vickers indenter or Knoop indenter, into a surface of a cover glass and comparing easiness of occurrence of cracks occurred from the indentation. However, there is a case where superiority or inferiority of the cover glass evaluated by the above-mentioned method does not always correlate with superiority or inferiority of the cover glass in the actual drop fracture, and a measurement method for strength of a glass further appropriately reflecting the situation of the actual drop fracture has been demanded. Furthermore, there has been a problem that fracture pattern of a glass in the above-mentioned method does not always consist with the actual drop fracture pattern.
In actually giving impact to a cover glass in, for example, the case that users drop a flat panel display device by mistake, slow crack in which a glass cracks in relatively slow rate starting from a flaw penetrating a compressive stress layer may occur even in a chemically strengthened cover glass (such cracking manner of a glass is hereinafter referred to as “slow cracking”).
Such a slow cracking occurs under lower load or by drop from lower place, as compared with so-called edge cracking or spider cracking described hereinafter, and the slow cracking remarkably differs from the cracking conventionally been problematic, in terms of this point.
In the study to the slow cracking and the development of a cover glass resistant to the slow cracking, that have hitherto been made, it has been extremely difficult to reproduce the slow cracking. For example, it has been difficult to cause the slow cracking by pushing an indenter having relatively large tip angle, such as Vickers indenter, into a glass. For this reason, it was necessary that a considerable number of fabricated flat panel display devices are fractured by dropping them on the ground or the like, glasses accidentally causing slow cracking are then extracted from the cracked glasses and those glass are evaluated.
However, reproduction of slow cracking by dropping a flat panel display device that is an actual product on the ground leads to not only poor efficiency, but the waste of the flat panel display device itself. For this reason, it has been desired to reproduce slow cracking in a chemically strengthened glass at the stage before a flat panel display device becomes a product.